Extension of hair shader for fur

Two papers describe how to render animal fur using the "double-cylinder" method. It refers to the outer cylinder representing the cortex and the inner cylinder representing the medulla within a fur stand.

This method has become the industry standard for photorealistic fur. The most recent version of Houdini has released an extension of their hair shader using the double-cylinder method. Blender needs to extend its own hair shader using that method.